1. Field of the Invention
This invention relates to the field of switching system, more particularly relates to side-tone adjusting apparatus and method corresponding to a speaking path selection request.
2. Description of the Prior Art
There has been rapid developments in telephone switching systems in recent years. In general, the exchange of a switching system is coupled to a plurality of extensions comprised of telephone sets and telephone lines. The exchange operates a call sequence by selectively connecting a calling telephone set to a designated called telephone set. The called telephone set responds to the call in order to complete the connection of a speaking path between the telephone sets. The exchange also operates another call sequence by selectively connecting an incoming telephone line to the telephone set that responds to the incoming call. The telephone set responds to the call in order to complete the connection of a speaking path between the incoming telephone line and itself.
Generally, each telephone set has a side-tone adjusting circuit comprised of T-type bridge circuit in order to obtain a suitable side-tone during a speaking. The side-tone adjusting circuit adjusts the side-tone in responsive to a balanced impedance.
In conventional telephone switching system, the balanced impedance is fixed to one predetermined value during a manufacturing. The manufacturer determines the impedance value corresponding to an impedance value of a predetermined standard length of telephone line because a suitable side-tone is obtained by a relationship between the impedance of telephone line and the balanced impedance. Therefore, if the telephone set is connected to the central telephone line extending from a distant central office, the quality of side-tone is debased because the line impedance increases. If the telephone set is connected to the PBX's telephone line extending from a close PBX, the quality of side-tone is also debased because the line impedance decreases. The more a difference between the actual line impedance and the predetermined line impedance increases, the more the quality of side-tone is debased. Thus the present system is inadequate when the system is adapted to connect a plural kinds of telephone lines.